1. Field of the Invention
The present invention relates generally to an apparatus for reproducing information recorded on an optical disc, and more particularly to an information reproducing apparatus of this kind which performs the processing for optically reading and reproducing information recorded on an optical disc being rotated, while correcting the time base of the read information.
2. Description of the Related Art
Attempts have been made for reproducing digital information recorded on an optical disc in a high quality, particularly in the field of audio-related applications. One of such attempts involves a technique of reproducing a read signal at a high resolution. Specifically, a read signal is sampled at a high rate and converted to a digital signal using a large number of quantization bits. Another attempt involves an extension of a frequency band in which a read signal is reproduced. In other words, the latter attempt aims at faithfully outputting an audio signal in a high frequency band, as well, which has been essentially ignored.
Other than the attempts mentioned above, there are also indispensable factors for high quality reproduction. One of these factors is a so-called time base correction, i.e., a reduction in fluctuations (jitter) of a read signal on the time base. In the prior art, such time base correction has also been employed, and a variety of approaches have been proposed therefor.
However, a majority of such proposals rely on an idea of macroscopically performing the time base correction, and tend to treat lightly microscopical time base correction. For this reason, a jitter component escaping from the macroscopic time base correction is likely to remain and affects the quality of reproduced information. Therefore, there is yet room for improvement in the time base correction. Particularly for audio devotees who continue insatiable pursuit of ever higher sound quality, even a slight improvement in sound quality is extremely useful, so that a search for complete accomplishment of the time base correction is important.
Specifically, a phenomenon has been found, wherein in spite of a small amount of jitter in an RF signal, i.e., a read signal, when this RF signal is converted to an EFM signal, which is a predetermined encoded signal, more jitter is produced in the EFM signal. Conventionally, a synchronization signal (reproducing clock signal) is recovered from a read signal, and a spindle servo is performed for rotating a disc based on the synchronization signal. However, since this synchronization signal is generated from the EFM signal as mentioned above or a signal equivalent thereto, the jitter component induced during the conversion from the read signal to the EFM signal affects the time base correction using a spindle motor, by way of the synchronization signal. In addition, the jitter component introduces a problem of supplying an unnecessary driving voltage to the motor so that a larger amount of electric power is consumed. Further, if excessively large frequency difference and phase difference are found between the synchronization signal and a reference signal when a spindle error is produced, beats occur between these signals, and a signal processing unit located at a rear stage to process signals based on the synchronization signal is burdened with a larger amount of digital processing, resulting in an increase in so-called digital noise.
Consequently, such microscopic troubles adversely affect the time base correction for a read signal, and in its turn prevent reproduction of high quality information.